FS#580 - Detect when on battery power and handle it gracefully

User reported the CP800 failed to print when on battery power, turns out one of the status fields changed causing us to fail. I believe that's resovled for the older SELPHY CP models, but the newer ones (including the CP790) and the ES series are likely to have a similar problem.

Not sure what can be done about this without getting the battery and experimenting...